It has been tacitly assumed that in normal persons the blood pressure and the volume of the pulse are equal in the right and left brachial and carotid arteries, and as a corollary, that sphygmic inequality in these arteries is evidence of disease of the aorta, provided that peripheral causes of partial arterial occlusion, such as encroachment of the clavicle on the subclavian artery, cervical ribs, thrombosis, embolism, and arteriosclerosis, as well as developmental anomalies of the aorta and its main branches, can be excluded. In other words, the occurrence of a lower pressure and smaller pulse on one side is taken to mean that some pathological process, such as syphilitic aortitis, has brought about narrowing of a main branch of the aorta at its origin. This is illustrated by the following case. Case 1. A man 45 years of age complained of nocturnal cardiac dyspnea, weakness, and air hunger on exertion. Physical examination disclosed the Corrigan pulse, selective left ventricular hypertrophy, and characteristic auscultatory signs, of pure aortic regurgitation. Except for a loud aortic systolic murmur, there were no indications of enlargement of the ascending or transverse portions of the aorta, but the innominate artery was very accessible in the retro-manubrial space, and there was parasternal dullness both to the right and left in the first intercostal space. The carotid pulses were of equal volume, but the pulse in the right subclavian and its branches was at all times strikingly smaller than that in the left. The brachial arterial pressures were recorded on two occasions only ( Figure 1A), partly because the patient contracted pneumonia immediately after his admission to the hospital, and was too ill thereafter to be disturbed unnecessarily. Additional features of the case were premature senility, generalized arteriosclerosis, absence of any history of rheumatic fever, and a strongly positive blood Wassermann reaction. Obviously the patient had syphilitic aortitis with aortic regurgitation, and the relatively low right brachial pressure made one suspect that the mesarteritis had extended throughout the innominate and partially obstructed the orifice of the subclavian. This suspicion was confirmed by postmortem examination, which showed that the diameter of the right subclavian orifice had been reduced one-third. Unfortunately, not all cases are so satisfactory. To begin with we shall exclude from consideration manifest aneurysm of the aorta, together with the hydrodynamic and mechanical alterations peculiar to aneurysm, and confine our attention to lesions which present genuine diagnostic difficulties with respect to etiology and pathological physiology and anatomy. The following cases illustrate some of the principal aspects of the problem.Case 2. A man 49 years of age without complaints referable to his heart presented the usual signs of aortic regurgitation, except that his pulse, although possessed of the requisite volume, exhibited a prominent anacrotic interruption which effectively obscured whatever ...